Commitments and Contingencies (Changes in Environmental Liability) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accrual for Environmental Loss Contingencies [Roll Forward]
Balance at Beginning of the Period	$ 8,490	$ 9,441	$ 12,770	$ 16,761	$ 21,651
Additions to Accrual	0	0	0	7,700	0
Expenditures for Remediation	1,900	951	3,329	11,691	4,890
Balance at Period-end	$ 6,590	$ 8,490	$ 9,441	$ 12,770	$ 16,761